1. Field of the Invention
The present invention relates to a charger for charging an image carrier, an image forming apparatus of electrophotographic type incorporating the charger, and a charge control method for controlling charging of the image carrier.
2. Description of the Related Art
In a conventional image forming apparatus including an electrophotographic machine, such as an electrostatic copier, a printer, and facsimile, an image is formed as follows. First, after the surface of a photoconductor is uniformly charged by a contact charger, an electrostatic latent image is formed on the surface of the photoconductor by an exposure unit. Then, the electrostatic latent image is developed by a developing unit into a toner image and transferred directly or through an intermediate transfer member onto a transfer material such as a paper by a transfer unit. Finally, the toner image on the transfer medium is fixed by a fixing unit.
There is known, as a conventional image forming apparatus, one provided with a charger that brings two charging members (primary and secondary charging members) into contact with a rotating photoconductor so as to uniformly and satisfactorily charge the surface of the photoconductor (see, e.g., JP-A-2005-331846). The charger of this image forming apparatus uses the secondary charging member provided on the downstream side in the rotational direction of the photoconductor to eliminate irregular charge occurring when the primary charging member provided on the upstream side in the rotational direction of the photoconductor is used to charge the surface of the photoconductor, thereby achieving uniform charge of the photoconductor.
However, in the contact type charge, there may be a case where residual toner particles passing through a cleaner of the photoconductor or other foreign matters are adhered to the charging member to contaminate it, causing irregular charge. In order to cope with this problem, in the charger disclosed in JP-A-2005-331846, at the time of image formation, a voltage whose absolute value is higher than the absolute value of a discharge start voltage Vth1 of the primary charging member is applied to the primary charging member and a voltage whose absolute value is lower than at least the absolute value of a discharge start voltage Vth2 of the secondary charging member is applied to the secondary charging member. Further, at the time of cleaning when an image is not being formed, a voltage which has the same polarity as that applied to the primary charging member at the time of image formation and whose absolute value is lower than the absolute value of the discharge start voltage Vth1 of the primary charging member is applied to the primary charging member and a voltage which has the reverse polarity to that applied to the secondary charging member at the time of image formation and whose absolute value is lower than the absolute value of the discharge start voltage Vth2 of the secondary charging member is applied to the secondary charging member. This configuration enhances cleanability of the secondary charging member, thus making the surface potential of the photoconductor stable.
Then, consider that the cleanability of the secondary charging member is enhanced so as to make the surface potential of the photoconductor stable in the case where the charger disclosed in JP-A-2005-331846 is applied to an image forming apparatus provided with a transfer belt that contacts the photoconductor. However, in the charger disclosed in JP-A-2005-331846, at the time of cleaning, a voltage whose absolute value is lower than the absolute value of the discharge start voltage Vth2 of the secondary charging member is applied to the secondary charging member. When such a low voltage is applied to the secondary charging member, an electrical discharge does not occur from the secondary charging member to the photoconductor, with the result that the surface potential of the photoconductor becomes substantially 0 V. Therefore, a charged part and non-charged part are formed on the photoconductor contacting the secondary charging member. When the charged part and non-charged part are formed on the photoconductor, a difference is generated in an electrostatic adsorption force between the photoconductor and the transfer belt. As a result, a difference is caused in the moving speed of the transfer belt, causing banding.